1. Field of the Invention:
This invention relates to an amplifier circuit for amplifying an input signal, and more particularly to an amplifier circuit capable of suppressing an output d.c. offset voltage to a minimum.
2. Description of the Related Art:
Heretofore, there have been known various types of amplifier circuits. FIG. 3 of the accompanying drawings shows the structure of a common amplifier circuit. In FIG. 3, the amplifier circuit 1 is comprised mainly of an amplifier A3 and a negative feedback resistor R3.
In the circuit 1, a signal input terminal 2 is connected to the non-inverting input of the amplifier A3, and the inverting input of the amplifier A3 is connected to D.C. power 6 via a capacitor C1. The output of the amplifier A3 is connected to a signal output terminal 5 having a capacitor C3 and a resistor R6 connected thereto as the load. The negative feedback resistor R3 is connected across the output and the inverting input terminal of the amplifier A3, and a capacitor C2 and a resistor R7 are connected in parallel with the resistor R3.
A signal input at the input terminal 2 is supplied to the non-inverting input terminal of the amplifier A3, and is delivered to the loads from the output terminal 5 after having been amplified at a gain level determined by the ratio of the resistor R3 to the resistor R5. In this example, d.c. components of the signal are cut off by the capacitor C3, and a.c. components of the signal are applied to the resistor.
Assume that a current flowing through the resistor R2 is Ii, and a current flowing through the resistor R3 is If. Given that Ii and If are 0.5 .mu.A, the resistor R3 is 300K.OMEGA., the resistor R2 is 30K.OMEGA. and the voltage of the d.c. power supply 6 is Vref, a potential VinDC at the positive input terminal of the amplifier A3 is given by the following equation: ##EQU1## At this time, an output potential VoDC of the amplifier A3 is expressed by the equation: ##EQU2## Hence, the output d.c. offset voltage VoDCof is given as EQU VoDC-VinDC=165 mV.
As described above, in the conventional amplifier circuit, when the amplifier circuit is operated at a low voltage power, e.g. 1.5 V or thereabouts, the output d.c. voltage of the amplifier A3 suffers from many fluctuations even if the output d.c. offset voltage is 165 mV, thereby reducing the dynamic range of the circuit and decreasing the maximum output.
Particularly, when increasing the gain level, a plurality of the transistors set forth in the above are connected in series to form a multistage amplifier in many cases. Such a circuit configuration, however, causes the foregoing offset voltage to be amplified at each stage, reducing effective amplification. Further, in order to prevent such a disadvantage as mentioned above, the amplifier circuits may be connected to each other by way of a capacitor. With such a structure, the d.c. components may be eliminated at the input side of each amplification stage, so that the amplification of the offset voltage is prevented. Since the capacitor occupies a much wider space than other elements, it becomes impossible to integrate the amplifier. Therefore, the structure accommodating the capacitor is not suitable for integrated circuit type amplifier. In addition, in an apparatus for reproducing an audio signal, it is necessary to prevent the attenuation of signals in the low-frequency band in the amplifier circuit in order to reproduce the signal with a high fidelity. For this reason, a large capacitor is required in the amplifier for connecting between circuits, which renders the integration of the amplifier circuit difficult.
This invention is aimed at overcoming the aforementioned drawbacks in the related art, and an object of this invention is to provide an amplifier circuit capable of suppressing an output d.c. offset voltage to a minimum, and having a greater stability.